Why use an electron microscope?
A traditional light microscope uses visible light
to resolve an image. The wavelength of visible light ranges from
about 400-700 nanometers. These wavelengths give the light microscope
physical limitations of 500-1000 times magnification and 200 nanometer
resolution. Electrons have both particle and wave-like properties.
When accelerated through a voltage, electrons can have a wavelength
on the order of 0.1 nanometers. This allows much higher resolution,
and magnification. The scanning electron microscope (SEM) that
is used in this program is capable of 20,000 times magnification
and about 10 nanometer resolution.
General SEM Design Features
Below is a simple schematic drawing of an SEM:
(Image courtesy of Museum
of Science)
A SEM functions in a similar way to a
traditional light microscope. An electron gun generates
electrons, which are then accelerated by an electric potential.
There are then condensing lens that collimate the electrons
into small beam. While lenses in an optical system are
made of curved pieces of plastic or glass, in a SEM a
lens is made by a coil of current-carrying wire. This
coil makes a magnetic field which exerts a force on the
moving electrons.
After the condensing lens is another coil
which scans the beam of electrons over the sample. There
is a final objective lens that focuses the beam to a small
spot on the sample. The electron beam hits the sample
and causes secondary electrons to leave the sample. The
secondary electrons hit a detector which generates an
electrical signal. This electrical signal is processed
by a computer into an image. All the SEM components are
contained inside a vaccum chamber to minimize electron-gas
interactions.
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